In processes involved with molten metals, glasses, slags, salts, etc., it is necessary to protect structural components from the action of these high temperature molten materials. It is conventional practice to interpose some type of relatively inert material between the structural components and the molten mass. Typically, this relatively inert material (or liner) is a ceramic paint often containing aluminum oxide, silicon dioxide, silicates, and sometimes boron nitride. Other liners are component pieces of the liner material. Many of these liner materials are not completely chemically inert and, additionally, since thin layers are used, can be easily mechanically damaged. Reaction between the molten material and the liner is particularly common when the liner contains silicon dioxide or silicates. The prior art liners must be frequently replaced as by recoating or relining. When monolithic liners are utilized, these are relatively difficult to fabricate into the complex shapes that are required for some applications.
The difficulty in fabricating solid boron nitride shapes is well known. Generally such shapes have only been obtained by hot pressing of boron nitride powder to yield high density material. The boron nitride must be hot pressed at very high temperatures (typically above 1800.degree. C.) into round logs from which the desired shape must be machined. Rectangular shapes have been produced but with some difficulty. Also, the length-to-diameter ratio is typically required to be less than one in order to get uniformity of densification. Such hot pressing limitations mandate that intricate shapes cannot be produced directly. Because of the cost and difficulty in achieving uniform and adequate densification for large parts, cylindrical logs are limited in diameter and length to about fourteen inches.
Other materials in common use for inert ceramic applications include aluminum oxide and zirconium oxide. These materials are not easily machined, even less so than the boron nitride. Additionally, the production of solid shapes requires either hot pressing or high temperature (over 1500.degree. C.) pressureless sintering. Therefore, their use for complex shapes is extremely limited.
Ceramic materials have many other uses, including applications where the material needs to achieve certain desired qualities. These qualities include high lubricity and good electrical resistance, high thermal conductivity, good mechanical properties and good chemical properties. For many of the applications, these properties must be available at both ambient and high temperatures in most any atmosphere. For these applications, the materials of interest are aluminum oxide, zirconium oxide, or boron nitride. However, the difficulty of fabricating these materials into complex shapes restricts their use significantly.
References that may have relevance to the present invention are U.S. Pat. No. 2,763,620 issued to J. Bugosh on Sep. 18, 1956; 2,915,475 issued to J. Bugosh on Dec. 1, 1959; 3,031,418 issued to J. Bugosh on Apr. 24, 1962; 3,108,888 issued to J. Bugosh on Oct. 29, 1963; 3,141,786 issued to J. Bugosh on Jul. 21, 1964; and 4,314,827 issued to M. A. Leitheiser, et al on Feb. 9, 1982.
Accordingly, it is an object of the present invention to develop a clay-like material that can produce a ceramic for use in contact with molten metals, glasses, slags, salts, plastics and the like that has a chemical resistance to reactions with these materials.
Further, it is an object of the present invention to develop a clay-like material which, upon drying, exhibits good lubricity as well as good electrical, thermal and mechanical properties both at ambient and high temperature.
Another object of the present invention is to develop a pressureless-sintered clay-like material that can be readily fabricated into the shapes that are required for a ceramic in applications wherein the ceramic material is in contact with molten metals, glasses, slags, salts, plastics, and similar materials.
A further object of the present invention is to provide a pressureless-sintered boron nitride-containing ceramic in near-to-net shape for use in applications wherein contact is made with molten metals, glasses, slags, salts and similar materials, and for applications where such a ceramic needs to have high lubricity, high electrical resistance, high thermal conductivity and good mechanical strength both at ambient and elevated temperatures.
It is also an object of the present invention to provide a method for the preparation of boron nitride containing clay-like material useful for producing pressureless sintered boron nitride-containing bodies that can be fabricated into intricate shapes by conventional fabricating processes.
These and other objects of the present invention will become apparent upon a consideration of the detailed description hereinafter.